1. Field of the Invention
This invention relates to an optical information disc apparatus, and more particularly to improvements to an optical information disc apparatus having an auto-focusing servo system.
2. Description of the Prior Art
When information is being recorded on an optical information disc apparatus, holes called pits are made in a metal film formed on a rotary disc by modulating the power of a laser beam according to the information, and the information is read from the disc by determining the amount of light reflected from the disc depending on the presence and absence of pits by collecting and shining a weak laser beam on the metal film containing the information. In a video disc apparatus which is used solely for reproduction, information is read out by shining a laser beam onto a replica made from the original disc through a similar process to that applied to conventional LP records.
For an optical information disc apparatus, one of the basic techniques for controlling the apparatus is auto-focusing, by which a converging lens is moved according to the upward and downward movements of the optical disc while the information is being reproduced so that the pits in the surface of the disc are always within the depth of focus of the laser spot. Several methods for an auto-focusing servo system applied to an optical disc apparatus have been proposed, each of which is characterized by a method of detecting the focus error by the variations of the light reflected from the surface of the disc. For instance, U.S. Pat. No. 4,293,944 disclosed an auto-focusing method using the astigmatic optical system shown in FIG. 1.
In FIG. 1, a laser beam 2 emitted from a laser source 1 such as a semiconductor laser, passes through a beam splitter 3 and a converging lens 4 and is focused onto a information track 6 of a disk 5. The reflected laser beam containing the information from the information track 6 passes through the converging lens 4, is reflected by the beam spliter 3 and is focused in only one plane by a cylindrical lens 7 so that it is converted into a non-regular beam. Accordingly, the cross-section of the reflected laser beam is wider in the X-direction at a point 8 (the point at which the reflected laser beam is focused by the converging lens 4 and the cylindrical lens 7), and it is wider in the Y-direction at a point 9 (the focus of the converging lens 4). The reflected laser beam at an intermediate point 10 (a point which is substantially at the mid-point between the points 8 and 9) exhibits a circular cross-section which has substantially the same dimensions in the X- and Y-directions. If the disk 5 moves in the direction in which it comes closer to the converging lens 4, the cross-section of the reflected laser beam at the point 10 becomes wider in the X-direction. Conversely, if the disk 5 moves in the direction in which it goes away from the converging lens 4, the cross-section of the reflected leser beam at the point 10 becomes wider in the Y-direction.
FIG. 2 is a schematic block diagram of an arrangement in which a photodetector 11 having four photocells 11a-11d is placed at the point 10 shown in FIG. 1. The signals of two 11a and 11c of the four photocells of the photodetector 11 are added by an adder 12, and the signals of the other two photocells 11b and 11d are added by another adder 13. The outputs from the two adders 12 and 13 are input to a differential amplifier 14. The output from the differential amplifier 14 is input to a driver 15 for the converging lens 4. The driver 15 moves the converging lens 4 either towards the disk or away from it in response to the output from the differential amplifier 14. More specifically, when the laser beam is exactly focused on the disk 5, the cross-section of the reflected laser beam on the photodetector 11 becomes a circular pattern which has equal dimensions in the X- and Y-directions as indicated by a solid line (a), so that the light intensity is evenly distributed, and the output of the differential amplifier 14 is zero. Therefore, the converging lens 4 is not moved. If the disk 5 moves so that it comes closer to the converging lens 4, the cross-section of the reflected laser beam becomes wider in the X-direction as indicated by the dashed line (b). Consequently, the output of the differential amplifier 14 becomes negative, and the converging lens 4 is moved away from the disk 5. Similarily, if the disk 5 moves away from the lens 4, the cross-section becomes wider in the Y-direction as indicated by the dashed line (c), the output of the differential amplifier 14 becomes positive, and the converging lens 4 is moved closer to the disk 5.
Other known auto-focusing methods include one using an off-center auxiliary beam in which, in addition to the main laser beam for reading out information, an auxiliary beam for detecting focusing errors is emitted through the converging lens at a position away from its optical axis so as to detect information based on the position of the reflected auxiliary beam containing information about the up-and-down movements of the disc; another in which masks are arranged asymmetrically about the optical axis within the area of the reflected light with a sensor divided into two parts near the point at which the image is formed, and so on.
However, in prior art optical disc apparatuses the converging lens is first fixed at a predetermined position away from the disc surface before starting to read information from the disc, and then it is moved toward the disc by energizing the voice coil of the converging lens using the output of a ramp generator while the auto-focusing servo system is kept on, so that the optical disc apparatus is induced into an auto-focusing operation. During the above process of moving the converging lens, the signal output from the differential amplifier 14 detecting focus errors varies along an S-curve as is well known. In a conventional apparatus, since the auto-focusing servo system is made to start the focusing operation when the converging lens is located at the base of the S-curve characteristic, a non-linear servo-operation occurs, causing a limiting cycle to be introduced depending on the time constant of the compensating circuit of the servo system, or the characteristics of the lens actuator, and this poses the problem of inducing low frequency vibrations in the auto-focusing operation.